Ionization smoke detecting device

ABSTRACT

An ionization smoke detector having a single ionization chamber, a pair of spaced electrodes in said chamber, a grid electrode between said spaced electrodes and a radioactive source within the chamber. A potential is applied between the spaced electrodes and means are connected between one of the spaced electrodes and the grid electrode to detect potential changes in the grid produced by the presence of smoke.

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States Patent Sasaki HONIZATION SMOKE DETECTING DEVICE Inventor: KojuSasaki, Tokyo, Japan Nittan Company Limited, Tokyo, Japan Filed: April13, 1971 Appl. No.: 133,767

Related US. Application Data Continuation of Ser. No. 880,023, Nov. 26,1969.

Assignee:

Foreign Application Priority Data Dec. 24, 1968 Japan ..43/94l81 US. Cl...250/43.5 D, 250/44, 250/83.6 FT,

[56] References Cited UNITED STATES PATENTS 3,560,737 2/1971 Skildum..250/83.6 FT X 3,448,261 6/1969 Amiragofi ..250/43.5 D

Primary Examiner-Archie R. Borchelt Attorney-Eugene E. Geoffrey, Jr.

[57] ABSTRACT An ionization smoke detector having a single ionizationchamber, a pair of spaced electrodes in said chamber, a grid electrodebetween said spaced electrodes and a radioactive source within thechamber. A potential is applied between the spaced electrodes and meansare connected between one of the spaced electrodes and the gridelectrode to detect potential Int. Cl. "swig $2 in the grid produced bythe presence of Field of Search ..250/43.5 D, 44, 83.6 FT;

340/214, 227, 228.1, 237; 313/54 1 Claim, 4 Drawing Figures VOLTAGEPOWER AMPLIFIER AMPLIFIER ALARM IONIZATION SMOKE DETECTING DEVICE Thisapplication is a continuation of Ser. No. 880,023, filed Nov. 26, 1969.

This invention relates to an ionization smoke detecting device and moreparticularly to an ionization smoke detecting device having a singleionization chamber.

When a gas contained within a limited space is ionized by a radioactiveray from a radioactive material such as radium and a resultant ioncurrent is measured, such ion current would vary if the composition ofthe gas should vary. Therefore, this change of composition of the gascan be detected by detecting the change in the ion current. This is theprinciple of an ionization smoke detecting device.

A prior ionization smoke detector has two ionization chambers, one ofwhich is airtight while the other is exposed to the air. Both ionizationchambers include pairs of electrodes which are mutually connected in ativated by detecting this unbalance of the bridge circuit. Since suchprior device requires two ionization chambers, it is disadvantageous inthat two radioactive sources are required and the construction iscomplicated.

Therefore, an object of this invention is to provide an improvedionization smoke detecting device having a single ionization chamber.

According to this invention, an ionization smoke detector is providedwith a smoke sensing section composed of a single ionization chamberincluding a pair of facing electrodes, a grid electrode located betweensaid facing electrodes and a single radioactive source located near oneof said facing electrodes; A potential drift detected at the gridelectrode due to presence of smoke is amplified to activate an alarmdevice.

Other objects and features of this invention will be more clearlyunderstood by reading the following description with reference to theaccompanying drawings.

In the drawings:

FIG. 1 is a schematic diagram, partially in block form, of an ionizationsmoke detector according to the prior art;

FIG. 2 is a graph explaining operation of the device of FIG. 1;

FIG. 3 is a schematic diagram, partially in block form, of an ionizationsmoke detector according to this invention; and

FIG. 4 is a potential distribution diagram illustrating operation of thedevice of FIG. 3.

Referring to the prior art structure shown in FIG. 1, two ionizationchambers l and 2 are utilized and embody radioactive sources 3 and 4.The radioactive sources are each carried by one of the electrodes whilea second electrode diagrammatically illustrated in a U- shapedconfiguration is positioned in spaced relationship to the electrodecarrying the radioactive material. The electrodes of the chambers areconnected in series and a voltage V is applied to the series connectedchambers. The drain-source path of a field effect transistor 5 isconnected in series with a resistor 6, and this series circuit isconnected in parallel with the ionization chambers. The gate electrodeof the transistor 5 is connected to the junction of the two ionizationchambers. The ionization chamber 1 is airtight and will be referred toas the internal chamber. The ionization chamber 2 is exposed to air andwill be referred to as the external chamber.

In FIG. 2, curves A and B represent the voltage-ion currentcharacteristics of the internal and external chambers l and 2. Whensmoke is introduced into the external chamber 2, it modifies thevoltage-current characteristic and produces a curve such as illustratedat B. This is caused by the reduction of the ion current resulting fromabsorption of the radioactive rays by the smoke and adsorption of theions by the fine smoke particles which are heavier than ions andelectrons and therefore lack mobility. Because the voltage-currentcharacteristic of the external chamber 2 has been changed, the currentchanges from i to i. Furthermore, the voltage across both chambers ismaintained at a constant value with the result that the voltage acrossthe external chamber changes from v, to v',. Thus the voltage changeacross the external chamber 2 is Av vr-v and this voltage change appearsacross the resistor 6. At least a portion of the voltage across theresistor 6 is fed to an amplifier 7 and actuates an alarm device 8.

FIG. 3 illustrates a device in accordance with the invention. Thisdevice has a smoke sensing portion which comprises a pair of spacedelectrodes 11 and 12 with a grid electrode 13 disposed therebetween. Aradioactive source 14 is positioned in the vicinity of the electrode 11.The electrode 11 and the grid electrode are connected to a voltageamplifier 15 which in turn is connected through a power amplifier 16 toan alarm 17. A constant voltage source 18 is connected to spacedelectrodes 11 and 12.

In operation the voltage source 18 produces a voltage E across theelectrodes 11 and'12 and the potential distribution between theelectrodes is a substantially straight line as illustrated bythe line Hof FIG. 4. This occurs under a smoke free condition. When smoke isintroduced between the electrodes 11 and 12, the potential distributionchanges markedly as illustrated by the graph K in FIG. 4. Thus thepotential of the grid electrode 13 changes from e, to e and the changeis Ae =e,-e

The foregoing effect is generally referred to as a space charge limitingeffect, and the magnitude of the change Ae is greater with a lowervoltage E and with a larger ion current. Therefore, the grid 13 ispreferably located at a position which will afford the greatest voltagechange. The change in the voltage is amplified by the amplifiers l5 and16, the former having a relatively high input impedance, and the outputof the amplifier 16 is fed to an alarm device 17.

As described above, the smoke detecting device in accordance with theinvention comprises a single ionization chamber and can therefore beconstructed simply and utilizes a single radioactive source. Moreover,the signal produced by the presence of smoke can be directly amplifiedby a conventional voltage amplifier and without the need for acomplicated bridge circuit such as that shown in FIG. 1. The device inaccordance with this invention also provides improved operation,requires simple circuitry, and affords stable operation.

While only one embodiment of the invention has been illustrated anddescribed, it is apparent that alterations, modifications and changesmay be made without departing from the true scope and spirit thereof asdefined by the appended claims.

What is claimed is:

1. An ionization smoke detecting device comprising a pair of spacedelectrodes having means for applying a DC potential therebetween, meanssupporting said electrodes to permit the movement of ambient airtherebetween, a potential sensing grid disposed between said spacedelectrodes, a radioactive source positioned in the vicinity of one ofsaid electrodes, said DC potential and radioactive source producing asub-

1. An ionization smoke detecting device comprising a pair of spacedelectrodes having means for applying a DC potential therebetween, meanssupporting said electrodes to permit the movement of ambient airtherebetween, a Potential sensing grid disposed between said spacedelectrodes, a radioactive source positioned in the vicinity of one ofsaid electrodes, said DC potential and radioactive source producing asubstantially linear voltage gradient between said electrodes, a voltageamplifier having an input and output, means connecting one of saidspaced electrodes and said grid to the input of said amplifier andindicating means connected to the output thereof, said amplifier beingresponsive to voltage changes between said one electrode and said gridproduced by the production of a nonlinear voltage gradient between saidelectrodes upon the entry of smoke therebetween and actuating saidindicating means.